1. Field of the Invention
The present invention relates to a communication apparatus in a radio communication system which shares frequency bands with other systems.
2. Description of the Related Art
With the progress of the information-oriented society, communication means which users can use in daily life have increased dramatically in number, and their communication speeds have also increased considerably. In order to increase the communication speeds, defining a method of using media in a manner different from the conventional manner have been proposed. In most radio communications, specific bands has been limited to be used by specific applications, according to the related regulation. However, several bands have been open to a plurality of applications without the need for any license. Recently, a very wide band, such as a band for UWB (Ultra Wideband) system, has been determined to be opened, limited to low-power short-range communication, overlaping with other applications. In such a case, however, a given communication is likely to become an interference wave with respect to another application.
According to regulation concerning UWB, when using a frequency which is not very robust against interference waves, a given system is obliged to stop transmission upon detecting a radio wave from a system having a license for the UWB frequency, i.e., a system having priority of use. Such a method is called DAA (Detect And Avoid). The concept of DAA is included in the concept of cognitive radio communication in a wide sense. Cognitive radio communication is a scheme of adapting to an environment by sensing and recognizing the environment. If it is defined that a target to be recognized is the use state of frequencies in a surrounding area and the method of adapting to an environment is to stop transmission upon detection of transmission from a licensed terminal, this method is equivalent to DAA.
Such a method is required to protect a system having priority. On the other hand, an abrupt stopping of transmission affects the communication performance on the cognitive side. A radio transceiver for video transmission detects a communication state and issues a retransmission request on the basis of the detected communication state (see, for example, JP-A 2004-336729 (KOKAI)). A communication state includes the electric field strength and error rate of a received radio wave and their temporal changes.
JP-A 2004-336729 (KOKAI) discloses a method using the electric field strength of a radio wave as a method of detecting the superimposition of interference from another system on transmission from a self system. However, this reference does not disclose anything about what kind of change is detected as a temporal change in electric field strength and how to improve the detection performance as compared with the case of simply detecting an electric field strength.
A cognitive radio communication system stops transmission at a given frequency immediately when a terminal having a license starts transmission on the same frequency. At this time, in most cases, the notification of transmission stopping from the transmitter does not reach the receiver on time before the receiver demodulates the signal in the frequency. As a consequence, the receiver may erroneously demodulate transmission from the licensed terminal instead of a signal from the cognitive transmitter. When the receiver performs error correction decoding at a likelihood corresponding to bit energy, the error correction performance deteriorates. A system subjected to changes in transmit/receive behavior in accordance with causes of interference, e.g., a cognitive radio communication system, is required to detect the cause of deterioration instead of simply detecting deterioration in communication performance.